Left-leaning pundits and activists who cackle gleefully at the prospect that current controversies will seriously damage Rush Limbaugh’s media career display their own vast ignorance of the talk-radio industry.

Yes, El Rushbo’s weekend apology for crude comments about Georgetown law student Sandra Fluke demonstrate his own recognition that these remarks fell far short of the “broadcast excellence” he regularly promises his 15 million listeners.

But neither uproar from all quarters against Limbaugh’s language, nor the much-publicized phone call from President Obama to support Ms. Fluke in her silly face-off with the most popular talk host in radio history, will prevent those committed listeners (not to mention a host of curious newcomers to the show) from tuning in to Rush in the weeks ahead.

In other words, it hardly matters if 95 percent of the public disapproves of Limbaugh using terms like “slut” and “prostitute” in response to Ms. Fluke’s demand to Congress that insurance from her Catholic university must provide her with free coverage for all her contraception needs. Neither Limbaugh nor other leading talkers worry about the overall “approval ratings” that obsess politicians.

There’s an unbridgeable gap between the dynamics of conservative media and the imperatives of electoral politics. In order to succeed in radio, you don’t need to win a majority of Americans, or even a majority of Republicans, or even a majority of those who are listening at the specific time of your broadcast. In fact, a show that consistently commands 5 percent of the available, major-market audience will earn millions and count among colleagues as a spectacular success. Limbaugh himself, who often (but not always) dominates ratings around the country, almost never scores more than 10 percent of the big market listeners who tune in to some form of radio during his three-hour daily show. The leading metro areas each boast well over 50 radio stations, so a program that draws even a mildly disproportionate share of the audience on a reliable basis becomes an attractive proposition to advertisers and to programmers.

Talk radio, in other words, appeals to a niche audience—drawing only a small fraction of the public even with its most successful shows, but still connecting with millions of people. The secret involves the fact that nearly everyone in the country listens to radio regularly—with a weekly audience most recently estimated at a staggering 242 million. This means that Limbaugh need not appeal to progressives or moderates or apolitical sports fans in order to maintain his franchise: he can remain a media powerhouse with an exclusive audience of hard-core right wingers.

As it happens, Rush actually does reach far beyond the conservative base—as do other successful right-leaning shows, including my own. Market studies show that a full third of the more than 4 million Americans who listen regularly to my radio show identify themselves as Democrats. They tune in to argue (we bill the show as “Your Daily Dose of Debate”), or to hear what the other side is saying, or to feel outraged or, we hope, to be entertained and informed. One of the lessons that radio consultants regularly attempt to convey is to avoid panic at the receipt of indignant letters or emails that promise “I’ll never listen to your show again.” Such pledges usually last until the next day of broadcasting, when the offended party tunes in to see if you’re still just as incurably awful as expected.

After all the Sturm-und-Drang over Limbaugh’s contraceptive controversy, this anomalous feature of radio reality means that a near-record audience will listen to his show to see how he follows up on the most recent developments. Even the withdrawal of leading sponsors and threats of boycotts won’t undermine his potency or power. The disenchanted advertisers will either return when the smoke clears or else find themselves replaced by other companies eager to reach an impressive audience.

Why, then, did Limbaugh take the uncharacteristic step of posting a statement on his website declaring that he felt sorry about his intemperate and tasteless comments, complete with an unequivocal declaration that “I sincerely apologize to Ms. Fluke for the insulting word choices”?

I can’t offer inside information on El Rushbo’s motivations, but I strongly suspect that the apology stemmed not from wounds inflicted on his own interests, or pain visited upon Sandra Fluke, but from his undeniably damaging impact on the conservative cause.

On the hot issue of the Obamacare mandate that forces even religious charities to provide free contraception coverage as part of their insurance policies, Republicans could win the argument as long as the conversation focused on religious liberty and freedom of conscience. When the administration succeeds in shifting the discussion to access to birth control, we lose. If they’re able to frame an even more extreme narrative—that conservatives side with Catholic charities as part of an over-arching “war on women” —then we lose, big time. By attacking an articulate if sanctimonious law student as a “slut,” and suggesting that she post her sex “videos online so we can all watch,” Limbaugh made it vastly easier to characterize the conservative position as misogynistic and hateful.

Why should he care, if even more listeners tune in to his radio show for the next installment of troglodytic tastelessness?

Because, contrary to Rick Santorum’s dismissal of Rush as a mere “entertainer” (“an entertainer can be absurd,” Righteous Rick suggested), Limbaugh feels committed to conservatism, not just to his own success. By seizing angry attention in the midst of a presidential campaign from issues on which Barack Obama looks painfully vulnerable, Rush undermined Republican arguments and damaged conservative candidates, whether or not he undermined his own standing in the industry. To mitigate that damage, to change the subject to more promising and important issues, and not to protect his professional interests, that apology became not only appropriate, but absolutely necessary.